Getting Aggressive About Passive Design
Learning Objectives - After this course, you should be able to:
- Discuss how some passive ventilation systems work.
- Explain the energy savings from passive ventilation systems.
- Explain the advantages of mixed-mode ventilation systems.
How sad that air-conditioning, perhaps the definitive building technology of the 20th century-responsible for the appearance of more architecture than all of the "isms," genius practitioners, and political dictators put together-has increasingly become a dirty word among some architects. And how ironic that the more energy-hogging air-conditioning systems we build, the hotter the planet becomes.
In his 1969 book The Architecture of the Well-tempered Environment, Reyner Banham considered the air-conditioning unit a "portent in the history of architecture." But might we be on the cusp of a decline in its dominance? Although basic, time-honored design strategies such as exterior shading, interior thermal mass, operable windows, and careful site orientation have reemerged in the past decade under the guise of green architecture, passive or natural ventilation remain exceptions.
According to McGraw-Hill Construction's 2006 Construction Outlook, America built more than 150 million square feet of office space in 2005 alone, with the largest portion in Phoenix. We can safely assume that all of this space-and that in Phoenix in particular-came equipped with air-conditioning. This overwhelming evidence of our addiction to what Willis Carrier, the father of modern air-conditioning, called "man-made weather," has not prevented an adventurous set of architects and engineers from aggressively pushing for more passive design strategies.
They call it the "Windy City"
Devon Patterson, AIA, a principal with Solomon Cordwell Buenz (SCB) in Chicago, calls the implementation of passive ventilation strategies-basically, anything to reduce dependence on air-conditioning-a "forward-thinking" consideration. In the firm's design for the 69,000-square-foot Information Commons and Digital Library at Chicago's Loyola University, scheduled to open in November 2007, the need for energy efficiency that wouldn't sacrifice the building's transparency led the architects to knock on Matthias Schuler's door. Schuler, a mechanical engineer who runs the Stuttgart-based climate engineering firm Transsolar, assisted SCB in developing a double-skinned glass curtain wall as part of an integrated system of radiant slabs, underfloor ventilation, and operable windows that would result in smaller overall mechanical systems. Patterson says Chicago's extreme weather conditions-hot summers, cold winters-prevented an entirely natural scheme, but that so-called "mixed-mode" systems that combine conventional heating, ventilation, and air-conditioning (HVAC) with various levels of natural ventilation represent the best attempt to combat air-conditioning's prevalence.
"We were initially going to install some sunshades, but we found this was a better way to mitigate heat gain," Patterson says. Air enters the building off of Lake Michigan from the east through automatically operated clerestory windows on the glass curtain wall. It then moves across the interior to louvers at the top of the west wall, where it enters the 3-foot-wide cavity of the double-skinned curtain wall, a design-build point-supported glass system. This warmed air then exhausts via a natural stack effect through a large vent on the top of the building. There are basically two kinds of natural ventilation-the stack effect and wind. Loyola represents a combination of the two.
Loyola is a simple enough solution-a tunable glass box wrapped around a concrete structure-but complexity resides in its details. For one, the east wall includes interior low-E shades that form a mini-double-skinned facade when early morning sunlight might otherwise overheat the space. (Europeans often place these shades on the exterior, Patterson says.) Two systems sandwich the air of the interior, which consists mostly of open space for computer workstations. A raised-floor displacement ventilation system provides conditioned air designed to handle the first 8 feet of vertical space, as opposed to conditioning the entire 12 feet to the ceiling. By locating the air supply in the floor, the architects could install in the exposed poured-concrete, barrel-vaulted ceiling the plastic tubing needed for radiant cooling and heating. Vaulting not only contributed more surface area for the radiant system, it also optimized reflection for the efficient T5 fluorescent indirect lighting system.
Of course, chilled slabs lead to worries of condensation and indoor rain showers, but Transsolar's modeling showed that with a minimum of 67 degrees Fahrenheit for the slabs, there would be only 10 days each year where the slabs would be colder than the dewpoint. The building's conventional HVAC system easily accommodates dehumidification for these instances. The west facade, though, remains the lynchpin in the design. Early computational fluid dynamic (CFD) modeling of this side of the building showed air would flow over the top of the extended curtain wall and create a negative pressure zone on the backside that would pull air out of the wall's cavity. The curtain wall sits atop a trench, which pulls air in to facilitate the stack effect. In winter months, with the trench closed, warm air builds up in the wall cavity to help heat the building.